In the TDMA (Time Division Multiple Access) communication method utilized in the field of digital mobile radio communication, frame synchronization is required between a mobile station and a base station. However, because the distance between the mobile station and the base station as well as the path of radio wave propagation between the stations incessantly changes, the time required for the arrival of the radio wave also necessarily changes.
Therefore, when the period of burst data transmission at the mobile station is always set at a constant value T, a change in the distance between the mobile station and the base station or a change in the path of radio wave propagation between the stations results in, for example, a shortened or extended time required for the arrival of the radio wave. Consequently, the number of bits to be received in one burst data transmission period T at the mobile station decreases or increases by one bit, and an error occurs during decoding.
In order to prevent such a trouble, the radio wave propagation time is measured at the base station, and this data is provided to the mobile station, so that the period of burst data transmission at the mobile station deviates within the range of the pulse width tw of one data bit.
On the basis of the frame marker of the burst data transmission period T.+-.td having the period deviation td, the mobile station carries out, for example, speech encoding for each frame unit, and the encoded data is transmitted as the burst data at the predetermined period of burst transmission For the purpose of attaining such speech encoding and other processing, it is necessary that the period T.+-.td of the frame marker is maintained at a value which is n times (n: an integer) as large as the period Tc of the clock that determines the processing timing. That is, synchronization between them is required.
A prior art method for frame synchronization will now be described by reference to FIG. 5.
In FIG. 5, it is supposed that, in the period T.+-.td of an asynchronous frame marker AFM, T is selected to be T=20 msec, and td is selected to be 3.7 .mu.sec at a transmission rate of 270 kb/s and 20 .mu.sec at a transmission rate of 50 kb/s, while the period Tc of the clock CLK used for the speech encoding is selected to be 125 .mu.sec.
As shown at FIG. 5 (I), the leading edge of a first clock CLK applied immediately after the risetime of an asynchronous frame marker is detected, and this clock is used as a synchronous frame marker SFM. According to such a method, the number of the clocks CLK included between the consecutive synchronous frame markers SFM corresponds to the number of predetermined periods, that is, T/Tc=20/125=160 periods, when the consecutive plural asynchronous frame markers AFM(a) and AFM(b) are all present on the sam side (the side immediately before the risetime of the clocks CLK in FIG. 5) when the risetime of the clocks CLK is considered as the border on each side.
However, an extra clock CLK will be included to increase the number of clocks corresponding to 161 periods when, as shown in FIG. 5 (II), the leading one (a) of the consecutive asynchronous frame markers AFM is present immediately before the risetime of the associated clock CLK, and the trailing asynchronous frame marker (b) is present immediately after the risetime of the associated clock CLK.
On the other hand, the number of periods is only 159 which is less by one period than the predetermined value when, as shown in FIG. 5 (III), the leading one (a) of the consecutive asynchronous frame markers AFM is present immediately after the risetime of the associated clock CLK, and the trailing asynchronous frame marker (b) is present immediately before the risetime of the associated clock CLK.
Thus, the prior art frame synchronization method has had the problem that, when the asynchronous frame marker is present in the vicinity of the risetime of the associated clock CLK, the period of the synchronous frame marker SFM becomes larger or smaller by one bit depending on the position of the asynchronous frame marker.
The present invention solves such a problem, and its object is to provide a novel frame synchronization method in which the synchronous frame marker having a predetermined constant period can be generated regardless of the wave propagation time.